1. Field of the Invention
The present invention relates to transmissions having at least two shafts arranged parallel to each other, and in particular to a vehicle transmission for agricultural or utility vehicles.
2. Description of the Related Art
In transmissions used, for example, in agricultural tractors, torque frequently is transmitted over very long parallel shafts, which support the gears needed to provided various gear ratios. These shafts are designed to transmit large torques, and therefore must withstand correspondingly large bending moments. Undesirable bending deflection caused by vibrations or the radial forces applied by the gears can be avoided by correspondingly large shaft diameters, or by supporting the shafts in multiple bearings.
The bending load on a shaft that is supported in bearings only at its ends increases from the bearing to the center of the shaft. It is largest in the areas of the shaft at which radial forces are applied. This loading can be absorbed by a corresponding design of the shaft thickness, in which the diameter of the shaft increase from the bearings towards the center. However, thick shafts are costly, and increase the weight of the transmission. Furthermore, they occupy space that cannot then be used for other gearbox components.
Supporting a shaft on multiple bearings results in a statically indeterminate load. It requires very precise alignment of the bearings to each other, so that the bearings themselves do not create additional forces which would result in increased bearing loads. A very high degree of precision therefore is required in the manufacture and alignment of the bearings, which can be achieved only with difficulty. Such precision is particularly difficult to achieve if the bearings are mounted in different gearbox housings, and therefore cannot be machined as a unit.
A hydrostatic-mechanical power distributing transmission providing a continuously variable transmission ratio for vehicles, in particular for agricultural or utility vehicles, is a good example of a transmission of the type having parallel shafts. Such transmissions typically include a hydrostatic drive continuously variable between two extreme positions, with a variable flow and a constant flow positive displacement device, and a compound gearbox which includes at least two planetary gearsets.
Vehicle transmissions generally are used to allow the vehicle engine to operate so that its output torque follows as closely as possible the maximum power characteristic at the desired vehicle speed. In doing so, the transmission varies the torque as well as the rotational speed. Continuously variable drives have the advantage over mechanical multi-speed gearboxes that they can operate at optimum engine speed at every vehicle speed condition. There is no interruption in power during shifting, which contributes greatly to driving comfort, especially with a work vehicle under load. Such transmissions are particularly useful for agricultural and utility vehicles, which in addition to the vehicle drive frequently have at least one additional power take off shaft (PTO). The rotational speed of the typical PTO is controlled independently of the vehicle ground speed, which is difficult if the engine speed is constantly varying.
EP-A1-0 302 188 and DE-A-35 12 523 show examples of such continuously variable drives. In these transmissions, the power supplied by the engine is distributed directly to a first input shaft of a compound gearbox and to a second input shaft via a hydraulic drive. The hydraulic drive contains a constant power device with constant volume flow and a variable power device with variable volume flow. By adjusting the variable power device, the rotational output of the hydraulic drive can range from negative rotational speeds to positive rotational speeds. The output shaft of the hydraulic drive then is connected to the second input shaft of the compound gearbox. (While referred to as an "input" shaft, this second "input" shaft can act as either an input or an output shaft, depending on which gears are engaged and the operational position of the hydraulic drive.) The compound gearbox combines the torques and rotational speeds of the engine and hydraulic drive to produce a final output.
The compound gearboxes in these references contain a double planetary gearset whose two sun gears are arranged on the aforementioned second input shaft. The first input shaft of the mechanical drive is connected to the planetary carrier of one of the planetary gearsets and to the ring gear of the other planetary gearset. Each planetary gearset of the compound gearbox is provided with a separate output shaft, which is connected to two sets of mechanical drives which can be engaged by shifter clutches.
The two planetary gearset output shafts behave differently upon adjustment of the variable speed hydraulic device. As the variable speed device is adjusted upward, the speed of the first output shaft increases, while the speed of the second output shaft decreases. The reverse is true when the variable speed device is adjust downwards. The synchronous conditions necessary for smooth power transmission are attained at the limit of each speed ratio, so it is possible using this structure to have a largely continuous increase or decrease in rotational speed at the output shaft of the entire transmission.
To cover a wide range of operating conditions, the overall range of gear ratios should be as wide as possible. The individual control ratio at a given gearbox input speed is the ratio of the maximum to minimum output speed of the variable speed device. The total control ratio is then this individual control ratio multiplied by the control ratios of gear ratios in the mechanical gearset.
With increasing total control ratio, however, the maximum required proportion of hydraulic power increases considerably. The ratio of maximum hydraulic power to the total control of the transmission is equal to the individual control ratio minus one, and divided by two. Since the hydrostatic transmission of power is less efficient than mechanical transmission by gears alone, the individual control ratio should be kept as small as possible.
In addition, a compound gearbox of this type usually requires a larger radial space than a mechanical transmission of the same capacity. This means a power distributing transmission cannot be exchanged for a mechanical multi-speed gearbox in a given vehicle, which would otherwise be desirable to provide the largest flexibility.